The present invention relates to capacitive sensors and, more particularly, to a sensor in which a frame surrounding a sensing element is anodically bonded to a glass layer having a sensing electrode over a portion of its surface.
Anodic bonding is used to affix and seal glass wafers to adjacent semiconductor layers in a variety of capacitive sensing devices, such as pressure sensors, flow sensors and accelerometers. Anodic bonding occurs between a piece of sodium-containing glass and an adjacent semiconductor when the glass is biased to a large negative potential relative to the semiconductor at temperatures of a few hundred degrees C. This draws sodium ions within the glass away from the glass/semiconductor interface, leaving a thin depletion region. The electric field across the depletion region is so intense that it breaks bridging bonds in the glass and draws the resulting oxygen ions toward the semiconductor. The semiconductor is therefore oxidized near the interface and chemically bonds the semiconductor to the glass.
In capacitive sensors of this type, a portion of the glass layer which is not anodically bonded is coated with a thin film sensing electrode. In order to avoid arcing, this electrode is often maintained at the same potential as the semiconductor body during the bonding process. Thus, a depletion region is formed in the glass adjacent the sensing electrode, as well as adjacent the semiconductor itself, drawing oxygen toward the electrode. Applicant has discovered that this results in at least a portion of the electrode material being oxidized and adversely affects the accuracy and reliability of the finished sensor. In extreme cases, the electrode material is completely consumed by oxidation.
In addition, applicant has found that oxidation continues while the sensor operates because the electrodes are typically maintained at a higher potential than the glass. This gradually draws additional oxygen toward the interface and oxidizes the electrode even further. In an inertial sensor, such as a silicon-on-glass accelerometer, this causes the gap on either side of a movable sensing element to change over time and affects the electrostatic forces required to servo the sensing element back to its null position. Significant errors can be introduced into the output of the device in this way.
Examples of capacitive silicon-on-glass accelerometers susceptible to the foregoing effects are disclosed in: O'Brien et al. U.S. Pat. No. 5,205,171; and Warren, K., Journal of the Institute of Navigation, vol. 38, no. 1, pages 91-99, Spring 1991.
Therefore, it is desirable in many applications to provide a structure and a method for eliminating the deleterious effects of anodic bonding in capacitive sensors.